Batter – Preheat oven to 350F. Line an 8-by-8-inch baking pan with aluminum foil, spray with cooking spray; set aside.
In a large, microwave-safe bowl melt the butter, about 1 minute on high power.
Wait momentarily before adding the egg so you don’t scramble it. Add the egg, sugars, vanilla, and whisk until smooth.
Add the flour, cream of tartar, optional salt and stir until just combined, don’t overmix.
Turn batter out into prepared pan, smoothing the top lightly with a spatula; set aside.
For Sprinkling – In a bowl (same one used for the batter is okay), combine the sugar, cinnamon, and stir.
Using a small spoon, evenly sprinkle cinnamon-sugar mixture over the prepared pan. Looks like a lot but it sinks down and soaks in while baking.
Bake for about 24 to 25 minutes, or until done. A toothpick inserted in the center should come out clean, or with a few moist crumbs, but no batter. Place pan on top of a wire rack to cool for at least 15 minutes before slicing and serving. Bars will keep airtight at room temperature for up to 1 week, or in the freezer for up to 6 months.